Field of the Invention
The invention relates to a circuit substrate and an electronic device, and in particular, a circuit substrate which is disposed in an electronic device and on which an electronic component is mounted by soldering or the like.
Description of the Related Art
In an electronic device, various electronic components (electronic elements) from an IC to a resistor to a connector are mounted on a circuit substrate, and a method called surface-mounting is widely used to mount these electronic components on the circuit substrate. In surface-mounting, solder is printed on a mounting land of a circuit substrate, an electronic component is mounted on the circuit substrate so that a mounting terminal of the electronic component can be put on the mounting land, and then the solder is melted by heating with a reflow furnace. After the melted solder flows into a space between the mounting land and the mounting terminal, the melted solder is cooled to stick the mounting land provided on the circuit substrate and the mounting terminal of the electronic component together, whereby the electronic component is fixed to the circuit substrate, and at the same time, required electric connection between the electronic component and the circuit substrate is established.
In this surface-mounting, when solder is heated with a reflow furnace printed on a circuit substrate, a flux component contained in the solder vaporizes to generate gas in the solder, and the generated gas remains in the cooled and hardened solder without being discharged from the solder, causing a so-called solder void to be formed. When a solder void is formed, a satisfactory joint strength cannot be obtained at a solder joint, and also, when there is a temperature change, a crack is formed at a solder joint due to the solder void, resulting in further decreasing joint strength.
Moreover, when a solder void is formed, there is fear that the reliability of electric contention between an electronic component and a circuit substrate will decrease. In particular, the greater the soldering area of a mounting terminal, the larger the amount of solder, and the longer the distance from the center of a mounting land to an outer periphery of the mounting land. For this reason, gas generated by vaporization of a flux composition resists being discharged from solder, and a solder void tends to be formed.
Accordingly, there have been proposed various techniques to prevent or reduce formation of a solder void in solder joining by surface-mounting using a mounting terminal with a large soldering area. According to a technique described in, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2006-303173, in a circuit substrate device in which a component electrode of an electric component is soldered to an electrode land formed on a circuit substrate, an area corresponding to the component electrode on the mounting land is divided into a plurality of land areas by a solder resist with a predetermined width. Then, solder disposed in the plurality of land areas electrically and mechanically joins the component electrode to the land areas, and at this time, a gap connecting to outside the component electrode is formed between the solder resist and the component electrode between the land areas.
According to the technique described in Japanese Laid-Open Patent Publication (Kokai) No. 2006-303173, however, because an electrode land is divided into a plurality of areas by a solder resist having a predetermined width, the amount of solder in the plurality of areas is uneven. Particularly in the case of a terminal whose soldering area is large, the amount of solder in a peripheral area tends to be small due to surface tension of the solder. When the amount of solder in the plurality of areas is uneven, joint strength varies with the areas. When solder joint strength in an area where the amount of solder is relatively small thus decreases, satisfactory joint strength at solder joints over the whole electrode land may not be obtained. Moreover, in an area with a small amount of solder, it is feared that the reliability of electric connection will decrease due to high electric resistance.